This invention relates generally to sealing systems and more specifically to a sealing system for slurry pumps.
A typical slurry pump arrangement is shown in FIG. 1. It includes a centrifugal pump 2, having a pump housing or casing 20, supported on a base or pedestal 3, an impeller 18, and a shaft 16. The shaft 16 is supported by bearings 4 within a bearing housing 5. It is rotatable on the bearings but is fixed against axial movement relative to the housing 5.
Shaft 16 includes a free end 17 adapted to be connected to a drive motor (not shown). The bearing housing 5 with bearings 4 and shaft 16 is slidable relative to base 3 toward and away from pump casing 20 on rails 6. Bearing housing 5 includes a depending tab or arm 7. Base 3 supports a threaded shaft 8 provided with adjustment nuts 9. The tab 8 includes a bore surrounding the threaded shaft. The nuts are positioned on either side of the tab and are used to adjust the axial position of the bearing housing 7 relative to the pump casing.
Because the slurry material pumped contains particulate matter it is very abrasive. There is significant wear on the inner surface of the impeller casing usually near the inlet to the impeller 18. In some slurry pumps, a liner material is used to protect the inner surfaces of the impeller casing. Such a liner is shown in FIG. 2 and is generally designated 22. The presence of such a liner is not a part of the invention which is equally applicable to pumps without liners.
Slurry pumps are designed to accommodate the wear associated with operation. As previously described, the impeller shaft 16 is supported in a bearing housing 5 that is axially adjustable relative to pedestal 3. As the liner 22 or the inner surface of a pump casing wears within the pump cavity, the pump efficiency decreases. The spacing of the impeller to the casing is reestablished by axial adjustment of the position of the bearing housing 5 and consequently the shaft 16 to move the impeller 18 toward the inner surface of the casing.
Sealing systems have been used on slurry pumps for many years. A typical prior art sealing system 10 for a slurry pump is shown in FIG. 2. The prior art sealing system 10 includes an outer housing or stuffing box 12. The inner cylindrical surface of the stuffing box 12 defines a cylindrical bore surface extending through the stuffing box 12. An impeller shaft 16, attached to an impeller 18, passes through the stuffing box bore. The impeller 18 is situated within an impeller casing 20 with a casing liner 22 lining the inner surface of the casing 20. The stuffing box 12 is attached to the impeller casing 20. The liner 22 may be elastomeric or may not exist. It is, however, a common element in a slurry pump.
A slip-fit hardened shaft sleeve 24 surrounds a section of the shaft 16 and is fixed to rotate with the shaft. The inner cylindrical surface of the bore of stuffing box 12 and the outer cylindrical surface of sleeve 24 defines an annular gap 28. The annular gap 28 is filled with a packing material 30 to form a seal between the stuffing box 12 and the shaft sleeve 24.
The packing material 30 is normally woven from fibers in a square or rectangular section which can be cut into annular rings. The packing material 30 is prevented from moving axially out of the stuffing box 12 by a gland plate 96. The gland plate 96 is attached to the stuffing box 12 by a series of bolts 97 disposed in a circular pattern.
As described above, the impeller shaft 16 can be manually adjusted axially along its axis to compensate for wear of the impeller 18, liner 22 or inner surface of pump impeller casing 20. As the shaft 16 is moved axially, the impeller 18 is moved axially closer to the inner surface of the casing at the inlet to compensate for wear. At the same time, the shaft sleeve 24 is also moved axially relative to the packing material 30. Since the radial distance from the interface of the shaft sleeve 24 and the packing material 30 to the axis of the shaft 16 is approximately equal throughout the entire interface, the sealing ability of the packing material 30 will generally stay the same when the shaft 16 is moved axially along its axis.
Mechanical seals are also used to seal the shaft and casing of the slurry pumps. Mechanical seals eliminate the need to manually adjust the packing follower. A prior art mechanical seal 134 suitable for use in a slurry pump arrangement is shown in FIG. 3. The prior art mechanical seal 134 is located within a stuffing box 112 attached to the pump casing which includes an enlarged diameter cylindrical bore 111. Mechanical seal 134 places the packing material illustrated in FIG. 2 as the sealing element.
The mechanical seal 134 includes a mating ring 148 fixed for rotation with a sleeve assembly 124 by a pin 156. The sleeve assembly 124 is attached to the shaft 116 by a collar 152. An axially movable primary ring 150 is retained within a stationary gland plate 196 and is axially biased by a spring 154. The spring 154 biases the primary ring 150 toward the mating ring 148 to bring a seal face of the primary ring toward contact with a seal face of the mating ring.
Replacing the packing material 30 of the sealing system 10 for a slurry pump, as illustrated in FIGS. 1 and 2, with a mechanical seal 134, as illustrated in FIG. 3, can greatly improve the sealing capability of the system. However, such a direct replacement could cause some problems not previously present with the use of packing material as the sealing element. As noted earlier, axial movement of the impeller shaft may be required to compensate for impeller wear. With the use of packing material, axial movement of the shaft relative to the packing material generally will not change the sealing ability of the sealing system 10 since the radial distance from the interface of shaft sleeve and the packing material to the axis of the shaft is approximately equal through out the entire interface. However, with the use of a mechanical seal as a direct replacement for the packing material, an axial movement of the shaft may significantly change the sealing ability of the sealing system. As the shaft is moved axially inward into the axial impeller casing, the distance between the gland 196 and the mating ring 148 increases axially. This in turn increases the axial distance of the cavity in which the spring is situated relative to mating ring 148 and thereby relaxes the spring and reduces the bias of the primary ring 150 toward the mating ring 148. A reduced bias of the primary ring 150 toward the mating ring 148 may reduce the sealing ability of the mechanical seal 134.
The present invention is directed to an improved slurry pump sealing system that employs a mechanical seal arrangement to seal the pump shaft relative to the pump casing but accommodates axial shaft adjustment without compromising the seal functionality. In this regard, the mechanical seal assembly is fixed to the bearing housing and impeller shaft and moves axially with the bearing housing and shaft. The relative axial relationship between the rotating and non-rotating seal components remains unchanged regardless of the axial position of the shaft, impeller, and bearing housing.
To accommodate such movement of the mechanical seal assembly relative to the pump casing, the sealing system of the present invention includes a contraction assembly having one stationary member associated with the pump casing and one movable member associated with the seal chamber defining housing and seal assembly contained within the seal chamber. Movement of the shaft in a direction toward the pump casing to adjust the internal clearance between the impeller and pump casing causes the movable member to move relative to the stationary member to accommodate the reduction of the distance between the pump casing and shaft bearing housing. An interconnecting member forms a contraction joint between these separate members and is sealed to contain the fluids within the pump casing and seal chamber.